Microwave-enhanced chemical vapor deposition techniques are known in the art.
For instance, U.S. Pat. No. 5,223,308 (Doehler) relates to a microwave-assisted deposition apparatus that comprises a microwave generator and a rectangular microwave waveguide that are employed to provide an electromagnetic field of intense microwave energy in a space in which an elongated hollow tube is continuously moved. The disclosed apparatus applies a coating of silicon oxide, silicon nitride, or silicon oxycarbide to the elongated hollow tube. The elongated tube, which is formed of a synthetic resin (e.g., a nylon material), is used in automobiles as piping for hydraulic air conditioning systems in order to minimize the loss into the atmosphere of liquid coolant (e.g., Freon).
U.S. Patent Publication No. 2003/0104139 (House et al.) relates to an apparatus for depositing a plasma chemical vapor deposition (PCVD) coating on the interior of a hollow glass tube. The applicator comprises a waveguide that guides microwaves from a microwave generator to an applicator head. The waveguide has an elongated axis and, perpendicular to this elongated waveguide axis, a rectangular cross-section having a long axis and a short axis. A glass tube is positioned within the applicator head, and the applicator head is moved over the hollow glass tube along the longitudinal axis of the tube.
U.S. Patent Publication No. 2003/0115909 (House et al.) relates to an apparatus for depositing one or more glass layers on the interior of a hollow substrate tube. An activator space of a microwave applicator surrounds the hollow substrate tube. Microwaves generating plasma in the interior of the hollow substrate tube cause the glass-forming precursors to deposit silicon dioxide (SiO2) onto the interior of the substrate tube.
U.S. Pat. No. 6,849,307 (Breuls et al.) relates to an apparatus for manufacturing an optical fiber from a preform. An elongated vitreous substrate tube (e.g., including quartz) is coated on its interior cylindrical surface with layers of doped silica (e.g., germanium-doped silica). This can be achieved by positioning the substrate tube along the cylindrical axis of the resonant cavity and flushing the interior of the tube with a gaseous mixture that includes, for example, oxygen (O2), silicon tetrachloride (SiCl4), and germanium dichloride (GeCl2). A localized plasma is concurrently generated within the cavity, causing the reaction of silicon, oxygen, and germanium so as to effect direct deposition of germanium-doped silica (SiOx) on the interior surface of the substrate tube. As the deposition occurs only in the vicinity of the localized plasma, the resonant cavity (and thus the plasma) must be swept along the cylindrical axis of the substrate tube in order to uniformly coat its entire length. When coating is complete, the substrate tube is thermally collapsed into a rod having a germanium-doped silica core portion and a surrounding undoped silica cladding portion.
Thereafter, if an extremity of the rod is heated so that it becomes molten, a thin glass fiber can be drawn from the rod and wound on a reel. The glass fiber possesses a core portion and a cladding portion that corresponds to those of the rod. Because the germanium-doped core has a higher refractive index than the undoped cladding, the glass fiber can function as a waveguide for optical signals (e.g., propagating optical telecommunication signals). With respect to U.S. Pat. No. 6,849,307, the gaseous mixture flushed through the substrate tube may contain other components. The addition of hexafluoroethane (C2F6), for instance, reduces the refractive index of the doped silica.
Furthermore, the solid preform may be placed in a so-called jacket tube (e.g., including undoped silica) before drawing to increase the quantity of undoped silica relative to doped silica in the resulting glass fiber. Additional silica may also be applied via a plasma process or outside vapor deposition (OVD) process.
U.S. Pat. No. 6,849,307 is hereby incorporated by reference in its entirety.
The use of an optical fiber for telecommunication purposes requires that the optical fiber is substantially free from defects (e.g., discrepancies in the dopants concentration or undesirable cross-sectional ellipticity). Over great lengths of the optical fiber, such defects may cause significant signal attenuation.
The PCVD process must be uniform and reproducible because the quality of the deposited PCVD layers determines the quality of the optical fibers. In particular, the plasma generated in the resonant cavity should be rotationally symmetrical (i.e., around the cylindrical axis of the resonant cavity).
On the other hand, production costs would be reduced if a suitable large-diameter preform could be employed (i.e., more fiber lengths might then be obtained from a single preform). Unfortunately, increasing the resonant cavity's diameter to accommodate a thicker preform will cause the rotational symmetry of the plasma to deteriorate. Moreover, to generate a plasma sufficient for a large-diameter preform would require much higher microwave power.
In the PCVD apparatus disclosed in U.S. Pat. No. 6,849,307, energy from a source capable of generating microwaves (e.g., a klystron) is transferred to an annular resonant cavity so as to form a plasma zone in the interior of the substrate tube. In particular, microwaves are transferred from the microwave source to the annular resonant cavity via an elongated microwave guide. The microwave guide has a central longitudinal axis that is substantially perpendicular to the cylindrical axis of the annular resonant cavity.
In this kind of PCVD apparatus, an antenna present within the microwave guide is centered using one or more centering components. Such centering components, which are likewise positioned within the microwave guide, encounter and thus must be permeable to microwaves. The centering components enable the antenna to move along the longitudinal axis in the microwave guide and furthermore ensure that the antenna cannot touch the wall of the microwave guide.
The present inventors have found that centering component materials are susceptible to undesirable sparking. Moreover, such materials reduce the maximum microwave power supplied to the resonant cavity, which is attributable to their relatively low dielectric strength. In addition, the present inventors have found that the surface roughness of such centering component materials introduces minor air channels, which adversely affect the performance of the resonant cavity.
The present inventors have further established that if sparking occurs, the mechanical loads to which such materials are subjected are so heavy that cracking or even evaporation may ensue, causing damage to the resonant cavity and requiring eventual replacement of the resonant cavity.
It is desirable, of course, to achieve high rates of deposition of glass layers on the interior of the substrate tube. This requires high microwave power. Consequently, such centering components constitute an undesirable constraint for PCVD devices.